This invention relates to the support, grounding, and self-biasing of microcircuits, and, more particularly, to the support, grounding, and self-biasing of microwave monolithic integrated circuits.
A microwave monolithic integrated circuit (MMIC) is a microelectronic device used to process microwave signals having frequencies of from about 0.9 GHz (gigahertz) to about 100 GHz. In one example, a signal is input to a MMIC on an input microwave stripline, processed as by amplification of the signal in the MMIC, and output on an output microwave stripline. Such amplifier MMICs are used in communications satellites to boost the signal strength of microwave signals received from earth, prior to re-transmission.
A typical MMIC microcircuit assembly includes an electrically grounded metallic carrier and an electrically nonconducting substrate mounted to the carrier. The MMIC lies within a window-like via through the substrate, and is mounted directly to the carrier. The striplines are supported on the substrate, and electrically interconnected to the MMIC with wire bonds.
In an MMIC microcircuit assembly, an electrical ground is established as a reference. In the usual case, a circuit ground on the MMIC is contacted directly to the grounded metallic carrier. This grounding structure is effective for both high and low frequencies.
Power to the MMIC is provided from a power source, such as a solar power system in the case of the communications satellite. The power source provides power to precision power supplies, which in turn power the circuitry of the MMIC. Positive and/or negative (relative to the electrical ground) power supplies are provided as necessary for the circuitry of the MMIC.
Existing MMIC microcircuit assemblies work well, and are widely used. However, there is an ongoing need to reduce the size, weight, and complexity of MMIC-based systems, particularly those used for space applications. The present invention fulfills this need, and further provides related advantages.
The present invention provides a microcircuit assembly that permits elimination of a negative power supply through self-biasing of an integrated circuit. The self-biasing is achieved through a change in the architecture of the microcircuit assembly that adds very little weight. The result is a net reduction in size, weight, and complexity of the assembly. The approach of the invention is particularly useful for microwave microcircuit assemblies, such as monolithic microwave integrated circuits.
In accordance with the invention, a microcircuit assembly comprises an integrated circuit structure having a circuit ground, an electrical input to the integrated circuit structure, and an electrical output from the integrated circuit structure. The integrated circuit structure, such as a monolithic microwave integrated circuit (MMIC), preferably operates in the microwave range of from about 0.9 to about 100 GHz (gigahertz). The microcircuit assembly further includes a carrier having a carrier ground thereon, and a dielectric substrate having a top surface, a bottom surface contacting the carrier, and a capacitor interconnect extending through the dielectric substrate. The capacitor interconnect is in communication with the carrier ground of the carrier. There is a metallization on the top surface of the substrate, with the metallization including a substrate ground plane upon which the integrated circuit structure is affixed. The electrical input and output of the microcircuit assembly may be formed as striplines in the metallization. A capacitor is electrically connected between the substrate ground plane and the carrier ground on the carrier. The capacitor is preferably a thin-film capacitor disposed within a capacitor via through the substrate. Desirably, an electrical resistor, which may be one or more of a set of resistors deposited on the substrate, is electrically connected between the circuit ground of the integrated circuit structure and the carrier ground.
The present architecture provides two grounding paths between the microcircuit assembly and the carrier (DC) ground. One grounding path is through the capacitor connected between the substrate ground plane and the carrier ground, which is effective at relatively lower frequencies. At higher frequencies, this grounding path becomes less effective due to the reactances of the wire bonds used to interconnect the capacitor. The second ground path is the effective capacitance established between the substrate ground plane and the carrier, through the dielectric substrate. This second grounding path becomes more important at higher frequencies, supplementing the first grounding path through the capacitor.
With this arrangement, where the integrated circuit is isolated from the carrier ground except for the grounding paths through capacitive structures, the circuit ground of the integrated circuit may be connected to the carrier ground through a resistor, which effectively biases the circuit ground negatively. A negative bias on the associated portions of the integrated circuit is thereby established. The value of the negative bias may be established by selection of the electrical resistance of the resistor. To provide a range of resistances and thereby a range of negative bias values, two or more resistors may be formed on the substrate in an array. The integrated circuit is connected to any selected one or ones of the resistances with the appropriate wirebonds.
More generally, a microcircuit assembly comprises an integrated circuit structure having a circuit ground, an electrical input to the integrated circuit structure, an electrical output from the integrated circuit structure, a carrier having a carrier ground thereon, and a dielectric substrate having a top surface to which the integrated circuit structure is affixed and a bottom surface affixed to the carrier. The microcircuit assembly further includes a capacitor structure electrically connected between the circuit ground and the carrier ground on the carrier, and an electrical resistor electrically connected between the circuit ground of the integrated circuit structure and the carrier.
This approach provides grounding of the integrated circuit, while at the same time allowing a negative bias to be established in the integrated circuit structure without the use of a negative power supply. The result is reduced size, weight, and complexity of the electrical system. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.